Store-provided bags are perceived as brand vehicles by management, so they'd rather be on the safe side when deciding about which gauge to get, wether recycled bags from stocks might be good enough and so on. Purpose-bought bags? Unless you get the heavy duty stuff like someone planning to dispose of their hacked-up murder victims will be much less material per bag.

But the main issue with ubiquitous bags is that not all of them make it to disposal, und that just doesn't happen that much with bags specifically bought for that very purpose.